The Bloom helicase (BLM) gene product encodes a DNA helicase that functions in homologous recombination repair to prevent genomic instability.BLM is highly active in binding and unfolding G-quadruplexes (G4),which are noncanonical DNA structures formed by Hoogsteen base-pairing in guanine-rich sequences.Here we use single-molecule fluorescence resonance energy transfer (smFRET) to study the molecular mechanism of BLM-catalysed G4 unfolding and show that BLM unfolds G4 in two pathways.Our data enable us to propose a model in which the HRDC domain functions as a regulator of BLM,depending on the position of the HRDC domain of BLM in action:when HRDC binds to the G4 sequence,BLM may hold G4 in the unfolded state;otherwise,it may remain on the unfolded G4 transiently so that G4 can refold immediately.